1. Field of the Invention
This invention generally relates to a 3-level inverter and more particularly, to a 3-level inverter that will be suitable for converting a DC (direct current) to an AC (alternating current) in three operation modes.
2. Description of the Related Art
Inverters are known as power converters for driving AC motors, and 3-level invertors are particularly used for large-scale AC motors.
The 3-level inverter includes snubber circuits for preventing an excessive voltage from being applied to each switching device during the process in which four switching devices connected in series with one another shift from ON to OFF. As described in J-PA-7-312878, JP-A-8-182341 and JP-A-2000-341961, for example, the snubber circuit is provided to each switching device. The snubber circuits for use in the inverter include a charging/discharging system in which a capacitor and a resistor constitute a charging/discharging circuit, and a clamp system that clamps a voltage exceeding a voltage of a DC power source when such a voltage is applied to the switching device.
Since the snubber circuit is provided to each switching device in the prior art technologies described above, the number of components is great, the inverter cannot be easily rendered compact, reliability drops.
It is an object of the invention to provide a 3-level inverter capable of permitting switching devices to conjointly use snubber circuits without disposing the snubber circuit for each switching device.
In a 3-level inverter including first to fourth switching devices connected in series with one another with an AC output terminal being the center; first to fourth rectification devices respectively connected in reverse parallel relation with the first to fourth switching devices; a fifth rectification device connected to a DC neutral point as a junction point between first and second DC power sources connected in series with each other and to a junction point between the first and second switching devices; and a sixth rectification device connected to the DC neutral point and to a junction point between the third and fourth switching devices; the first and third switching devices and the second and fourth switching devices being ON/OFF controlled in a mutual conjugated relation; the object of the invention described above can be accomplished by a 3-level inverter that comprises a first snubber circuit for clamping a voltage applied to the first or third switching device when the voltage exceeds a power source voltage during the processing in which the first or third switching device shifts from ON to OFF; a second snubber circuit for clamping a voltage applied to the second or fourth switching device when the voltage exceeds a power source voltage during the processing in which the second or fourth switching device shifts from ON to OFF; and a third snubber circuit interposed between the DC neutral point and the AC output terminal, for constituting a charging/discharging circuit.
When constituting the 3-level inverter, the invetor may include, as the snubber circuits, first and second snubber circuits each comprising a series circuit of a capacitor and a resistor as main constituent elements, and a third snubber circuit constituting a series circuit of a capacitor and a resistor. In this case, the first snubber circuit is connected to a positive plate terminal of the DC neutral point and to a negative plate terminal of the second DC power source, and the third snubber circuit is connected to the DC neutral point and to the AC output terminal.
A snubber rectification device may be connected in this case in parallel with a resistor of each of the first and second snubber circuits. The anode side of the first snubber rectification device of the first snubber circuit is connected to a positive plate terminal of the first DC power source and the cathode side of the snubber rectification device of the second snubber circuit is connected to a positive plate terminal of the second DC power source.
To constitute each 3-level inverter described above, the first to fourth switching devices may be insulated gate type semiconductor switching devices, for example IGBT or MOSFET.
In the construction described above, the third snubber circuit retards the rise of the voltage applied to the switching device among the first to fourth switching devices that shifts from ON to OFF, the first snubber circuit clamps the voltage applied to the first or third switching device when this voltage exceeds the power source voltage during the process in which the first or third switching device shifts from ON to OFF, and the second snubber circuit clamps the voltage applied to the second or fourth switching device when this voltage exceeds the power source voltage during the process in which the second or fourth switching device shifts from ON to OFF. Therefore, the invention can prevent an excessive voltage from being applied to each switching device by using a smaller number of snubber circuits than the number of the switching devices, can render the inverter compact in size, can improve reliability and efficiency and can reduce the production cost.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.